This invention relates to polyvinyl butyral and more particularly to improvements in processes for synthesizing polyvinyl butyral.
Polyvinyl butyral polymer or resin is used in sheeting with glass for laminated glazings, in specialty surface coatings and as binders for paper coatings, toners and the like. Polyvinyl butyral compositions in these applications contain about 10 to about 30% by weight functional hydroxyl groups with the remainder being essentially vinyl butyral (hereinafter "PVB").
It is known to prepare PVB by reacting polyvinyl alcohol (PVOH) and butyraldehyde, usually in an aqueous medium under acidic conditions. This acetal reaction is initiated in the homogeneous phase at relatively low temperatures. When a certain amount of the hydroxyl groups of the PVOH are reacted with the butyraldehyde, PVB polymer precipitates out. The reaction is then completed in the heterogenous phase at elevated temperature. The PVB particles formed tend to agglomerate into clusters which cause reactor wall scale buildup. Leverages for particle or granule (clusters of particles) size control are agitation and reaction temperature during the homogeneous phase. There are practical limits, however, on the temperature range for the homogeneous phase of the reaction. Besides high viscosity, too low temperature undesirably adversely affects polymer properties. Reaction temperature above about 25.degree. C. causes reactor fouling and results in very coarse granule size. High agitator speeds are impractical in large size commercial reactors. As a result, significant PVB polymer (typically on the order of about 1-3% by weight) is conventionally discarded as oversize, off-grade material in commercial PVB manufacturing operations.
It is likewise known to use additives for various purposes during PVB synthesis, which additives are variously referred to in the prior art as emulsifying or dispersing or suspending agents or surfactants (hereinafter interchangeably referred to as "surfactants"). U.S. Pat. No. 2,345,946 is typical in disclosing dispersing agents in the acetalization process for preparing PVB to prevent excess agglomeration of PVB particles.